Geoscience Reference
In-Depth Information
the crust and the high argon-40 content of the
atmosphere indicate that the differentiation and
degassing has been relatively efficient. The pres-
ence of helium-3 in mantle magmas shows, how-
ever, that outgassing has not been 100% effi-
cient. The evidence from helium and argon is
not contradictory. Helium dissolves more read-
ily in magma than argon and the heavy noble
gases so we expect helium degassing to be less
effective than argon degassing. We also have no
constraint on the initial helium content of the
mantle or the total amount that has degassed.
So helium may be mostly outgassed and what we
see is just a small fraction of what there was. The
evidence for differentiation and a hot early Earth
suggest that much of the current magmatism is
a result of recycling or the processing of already
processed material. The presence of helium-3 in
the mantle suggests that a fraction of the magma
generated remains in the mantle; that is, magma
removal is inefficient.
reservoirs and the geophysical evidence for gross
layering suggest that differentiation and chemi-
cal stratification may be more important in the
long run than mixing and homogenization.
Fluids and small-degree melts are LIL-
enriched, and they tend to migrate upward. Sedi-
ments and altered ocean crust, also LIL-enriched,
re-enter the upper mantle at subduction zones.
Thus there are several reasons to believe that the
shallow mantle serves as a scavenger of incompat-
ible elements, including the radioactive elements
(U, Th and K) and the key tracers (Rb, Sr, Nd, Sm
and, possibly, Pb and CO
2
). The continental crust
and lithosphere are commonly assumed to be the
main repositories of the incompatible elements,
but oceanic islands, island arcs and deep-seated
kimberlites also bring LIL-enriched material to
the surface. Even a moderate amount of LIL in
the upper mantle destroys the arguments for a
primitive lower mantle or the need for a deep
radioactive-rich layer.
It is becoming increasingly clear that all
magma are blends of melts from an inhomoge-
nous mantle. In fact, the source of magma has
been described as a
statistical upper man-
tle assemblage,
or SUMA. Various apparent
inconsistencies between the trace element ratios
and isotopic ratios in basalt can be understood
if (1) partial melting processes are not 100%
efficient in removing volatiles and incompati-
ble elements from the mantle; (2) basalts are
hybrids or blends of magmas from depleted and
enriched components; and (3) different basalts
represent different averages, or different volume
sampling. In a chemically heterogenous man-
tle mixing or contamination is inevitable. Even
with this mixing it is clear from the isotopes
that there are four or five ancient components
in the mantle, components that have had an
independent history for much of the age of the
Earth.
Some components in a heterogenous man-
tle melt before others. Material rising from one
depth level advects high temperatures to shallow
levels, and can cause melting from the material
in the shallow mantle. It is not necessary that
material melt itself
in situ
. Cold but fertile sub-
ducted or delaminated material will melt as it
warms up to ambient mantle temperature; excess
Generalities
In most models of basalt genesis, it is assumed
not only that olivine and orthopyroxene are con-
tained in the source rock but that these are
the dominant phases. Petrologic and isotope data
alone, however, cannot rule out a source that
is mainly garnet and clinopyroxene. The eclogite
(garnet plus clinopyroxene) source hypothesis dif-
fers only in scale and melt extraction mechanism
from the fertile peridotite hypothesis. In the fer-
tile peridotite, or pyrolite, model the early melt-
ing components, garnet and clinopyroxene, are
distributed as grains in a predominantly olivine-
orthopyroxene rock. On a larger scale, eclogite
might exist as pods or blobs in a peridotite man-
tle. Since eclogite is denser than peridotite, at
least in the shallowest mantle, these blobs would
tend to sink and coalesce. In the extreme case an
isolated eclogite-rich layer might form below the
lighter peridotite layer. Such a layer could form
by crustal delamination, subduction or by crystal
settling in an ancient magma ocean. Melts from
such blobs or layers still interact with olivine and
orthopyroxene. If eclogite blobs are surrounded
by refractory peridotite they can extensively melt
upon adiabatic decompression, without the melt
draining out. The isotopic evidence for isolated
